The overarching hypothesis of this proposal that a prosthesis with actively powered knee and ankle joints will significantly enhance the mobility of many transfemoral amputees, and therefore significantly enhance their quality of life. Despite significant technological advances over the past decade, state-of-the-art transfemoral prostheses remain limited to energetically passive devices. The inability to deliver joint power significantly impairs the ability of these prostheses to restore many locomotive functions, including walking upstairs and up slopes, running, and jumping, all of which require significant net positive power at the knee joint, ankle joint, or both. Additionally, even during level walking, transfemoral amputees exhibit asymmetric gait kinematics, expend significantly more energy, and require a significant increase in hip torque relative to healthy subjects, which results in significantly increased socket interface forces. It is the hypothesis of this proposal that a prosthesis with actively powered knee and ankle joints will significantly enhance the mobility of many transfemoral amputees, both by diminishing the biomechanical disparity between transfemoral amputees and healthy persons during level walking, and by enabling forms of locomotion, such as ramp and stair climbing, not presently afforded by state-of-the-art devices. Significant recent advances in power supply and actuation for self-powered robots bring the feasibility of an actively-powered transfemoral prosthesis to the near horizon. Leveraging these recent advances, this proposal describes a means of developing a lightweight transfemoral prosthesis with the capability of delivering significant power at the knee and ankle joints over useful periods of time. Importantly, the work described in this proposal will form a sound clinical and technological foundation with which to inform subsequent decisions regarding the commercial development of the proposed and other actively powered prostheses. In order to form this foundation, the investigators propose the development of a self-powered active knee and ankle prosthesis prototype based on experimentally proven technology developed over the past six years, the development of a novel interface and control approach that behaves inasmuch as possible as a natural extension of the user, and the characterization of the biomechanical benefits of an active transfemoral prosthesis relative to state-of-the-art passive transfemoral prostheses and relative to healthy subjects.